This invention relates to elongated weak-link supercurrent devices and, more particularly, to controlling the propagation of mobile flux vortices in such devices.
In U.S. Pat. No. 3,676,718 granted to P. W. Anderson, R. C. Dynes and T. A. Fulton on July 11, 1972, there are described a variety of weak-link supercurrent devices (e.g., shift registers, memories) which are capable of sustaining one or more trapped magnetic field (flux) vortices which correspond to information bits. In an extended Josephson junction (SIS) device, that is, one which is long in one direction compared to the Josephson penetration depth .lambda..sub.J, the patent teaches that a vortex is induced by a spatial variation of the supercurrent J in which a positive supercurrent flows through the I-layer and into the contiguous superconductor to a depth of about .lambda..sub.L, the London penetration depth, then along the superconductor a distance of about 2.lambda..sub.J, thence through the I-layer again as a negative supercurrent into the opposite superconductor to a depth of about .lambda..sub.L and finally back to the point of beginning. Such a vortex supports a net magnetic flux of approximately .phi..sub.o = 2.07 .times. 10.sup.-15 Wb, the well-known flux quantum. As defined in the patent, the term vortex means an entity which includes both the circulating supercurrent J and the flux quantum .phi..sub.o induced thereby.
Once created, the patent states, a vortex prefers to position and distribute itself in a region so that a local minimum of the sum of the magnetic energy plus the Josephson coupling energy is established. These local minima can be thought of as magnetic potential wells in which vortices can be stored. Where a plurality of such potential wells are present in a single weak-link structure, it is possible to move the vortex from one such well to another by applying a force thereto as, for example, by applying a local current or magnetic field to a region near to the vortex. In such an arrangement, the presence of a vortex in a potential well may be viewed as a logical one in binary notation whereas the absence of a vortex would be a logical zero.
In contrast, if the structure in which the vortex is created has no local minima of energy over an extended length in the direction of propagation, then once set in motion the vortex will propagate at a velocity, and to a distance, determined by damping processes (e.g., single particle tunneling). This kind of structure could function as a transmission line on which information is carried in the form of a train of sequential vortices. As long as there are no voids (absent vortices) in the vortex train, the mutual magnetic repulsion maintains the vortex ordering. If, however, the train is encoded by eliminating some vortices to represent logical zero, then vortices near the void would tend to drift and the ordering of the vortices would be lost. To maintain ordering one solution suggested by the prior art is to construct the transmission line with periodically spaced potential wells which serve as memory locations for the vortices.